<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var rudio = prompt("请输入圆的半径");
        var long = Math.PI * 2 * rudio;
        var area = Math.PI * rudio * rudio;
        if (!isNaN(rudio)) {
            document.write('圆的半径:' + rudio + '<br>');
            document.write('圆的周长:' + long.toFixed(2) + '<br>');
            document.write('圆的面积:' + area.toFixed(2));
        }
    </script>
</body>

</html>